I noticed on planet KDE this post on desktop
search. It mentioned not using xattr for metadata because some
filesystems don't support it. I'd say that most filesystems don't, iso
filesystems, NFS (depending on setup), http, ftp and xemacs don't. The
simple solution to all this in libferris is to virtualise xattr just
like the filesytem itself is virtual. So you store xattr in RDF when
the underlying filesystem doesn't allow it.

I should also highlight that the tagging mentioned in the post referenced
by the above post is already available and usable with libferris :)
You can attach arbitrary metadata to virtual filesystem objects, index
them and search based on that metadata. Indexing can be done in many
formats, lucene, postgresql, RDF using redland (db4, sqlite,
postgresql) or on an LDAP server.